Most current generation etch tools require the use of helium backside cooling in order to maintain wafer temperature within stable ranges. The method for achieving sufficient cooling usually involves the use of lipseal O-rings in order to contain the high pressure helium on the backside of the wafer. The lipseals function to secure the etch chamber electrode to the backside of the wafer in order to prevent leakage, which leakage leads to wafer photoresist burning. Lipseal design, material, and hardness must be optimized to produce the proper seal having a sufficient lifetime in a reactive plasma.
Elastopolymeric PTFE (polytetrafluoroethylene or Teflon.TM.) compounds are currently the most frequently used materials for use in O-rings.
One significant drawback in the use of lipseal O-rings, and more particularly O-rings fabricated from elastopolymeric PTFE compounds, is that wafers often stick to the lipseal during semiconductor processing. The problem is most notable when high power plasma processes are run. In such cases, a large amount of power (in the form of heat) must be transferred through the wafer, which consequently heats the O-ring. As a result, the lipseal O-ring often fuses to the backside of the wafer. When this occurs, the lipseal can be dislodged from its groove. The misplacement of the O-ring seal causes the next wafer placed into the etch chamber to be burned due to inadequate seal, or broken due to the force of the mechanical wafer clamp.
Seal stickage is also notable in cases in which a seal is placed under high mechanical pressure, such as a slit valve type of assembly frequently used in doors of various instruments. If the valve is activated open by a spring or gravitational method, the O-ring stickage may become severe enough to prohibit valve opening because the spring tension may not be great enough to overcome the adhesion of the door to the O-ring.
O-ring suppliers have tried various methods to prevent the occurrence of lipseal stickage, but to date, have not identified a reliable method to prevent its occurrence.